Some of the conventional sensors that are used in biomedical implants require powering through batteries and lead wires. For example, a prior wireless telemetry technique for measuring the pressure and temperature in a fetus, uses an implantable pill shaped transmitter that transmits in pulse interval modulation and is battery powered. An external receiver converts the received RF signal into a digital pulse stream which is decoded into pressure and temperature data. The transmission range for the transmitter is 3 to 6 feet.
A disadvantage with sensors that require powering through batteries and lead wire is that the embedded sensing circuit is always on and power dissipates in the biological tissue. This causes local heating and shortens the life span of the sensor. Additionally, sensors powered through batteries and lead wires require that at least the batteries be implanted in the body. This increases the possibility of infection due to the potential for leakage from the batteries. Furthermore, sensors powered by batteries and lead wires require shielding from moisture and the lead wires reduce the mobility of the person with the implantable sensor.
Other known sensors are remote powering and monitoring equipments. However these sensors are typically bench scale and not portable. In one known system for measuring induced vibrations in hip prosthesis, an implanted transmitter is a Resistance-Inductance-Capacitance (RLC) series resonant circuit that transmits in Pulse Code Modulation (PCM) format. The transmitter is inductively powered and an external receiver circuit for receiving the transmitted signal is a tuned amplifier. In another known system for measuring force and temperature in a hip prosthetic head, the implantable transmitter is a single Negative-Positive-Negative (NPN) transistor coupled to a one-turn loop antenna. The transmitter is inductively powered and the external receiver is a loop antenna with an integrated amplifier strapped around the leg.
The disadvantage with known sensors that are battery-less is the typical requirement that an external receiving circuit that consists of localized external protuberance be strapped to the body of a patient/user or that a distributed circuit approach be implemented in a wearable vest. Both of these approaches limit the freedom of motion of the user performing ordinary daily activities, such as jogging or swimming. Depending on the location of the sensor and the type of diagnosis, the external sensor worn on the user could also impact the self esteem of the user. Furthermore, some systems with battery-less sensors are designed for close range proximity, i.e. the external receiving circuit is placed in direct contact with the skin.